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PHYSICS CLASS XII

EMI & ACLast 11 Years Board Problems

Eleven years board problems

CBSE 2018

1. The teachers of Geeta’s school took the students on a study trip to a power generating station, locatednearly 200 km away from the city. The teacher explained that electrical energy is transmitted over such along distance to their city, in the form of alternating current (ac) raised to a high voltage. At the receivingend in the city, the voltage is reduced to operate the devices. As a result, the power loss is reduced. Geetalistened to the teacher and asked questions about how the ac is converted to a higher or lower voltage.

(a) Name the device used to change the alternating voltage to a higher or lower value. State one causefor power dissipation in this device.

(b) Explain with an example, how power loss is reduced if the energy is transmitted over long distancesas an alternating current rather than a direct current.

(c) Write two values each shown by the teachers and Geeta. (4)2.

(a) State the principle of an ac generator and explain its working with the help of a labelled diagram.Obtain the expression for the emf induced in a coil having N turns each of cross-sectional area A,

rotating with a constant angular speed ‘ ’ in a magnetic field , directed perpendicular to the axis of rotation.

(b) An aeroplane is flying horizontally from west to east with a velocity of 900 km/hour. Calculate thepotential difference developed between the ends of its wings having a span of 20 m. The horizontalcomponent of the Earth’s magnetic field is 5 X 10–4 T and the angle of dip is 300 . (5)

OR A device X is connected across an ac source of voltage V = V0 sin . The current through X is given as

I = I0 sin

(a) Identify the device X and write the expression for its reactance.(b) Draw graphs showing variation of voltage and current with time over one cycle of ac, for X.(c) How does the reactance of the device X vary with frequency of the ac ? Show this variation

graphically.(d) Draw the phasor diagram for the device X. (5)

CBSE 2017

1. Predict the polarity of the capacitor in the situation describes below :

CBSE 2019

1. (a) Define mutual inductance and write its S.I. unit.(b) A square loop of side ‘a’ carrying a current I2 is kept at distance x from an infinitely long straight wire carrying a current I1 as shown in the figure. Obtain the expression for the resultant force acting on the loop.

3. What is the direction of induced current in coil PQ and CD?

4. Define mutual inductance between a pair of coils. Derive an expression for the mutual inductance oftwo long coaxial solenoids of same length wound one over the other.

OR Define self-inductance of a coil. Obtain the expression for the energy stored in an inductor connected across a source of emf.

5. A device 'X' is connected to an ac source V = V0 sin t. The variation of voltage, current and power inone cycle is shown in the following graph :

(a) Identify the device 'X'.(b) Which of the curves A, B and C represent the voltage, current and the power consumed in the

circuit ? Justify your answer.(c) How does its impedance vary with frequency of the ac source ? Show graphically.(d) Obtain an expression for the current in the circuit and its phase relation with ac voltage.

OR (a) Draw a labelled diagram of an ac generator. Obtain the expression for the emf induced in the

rotating coil of N turns each of cross-sectional area A, in the presence of a magnetic field(b) A horizontal conducting rod 10 m long extending from east to west is falling with a speed 5.0 ms–1

at right angles to the horizontal component of the Earth's magnetic field, 0.3 × 10–4 Wb m–2. Findthe instantaneous value of the emf induced in the rod.

CBSE 2016

1. (i) When an AC source is connected to an ideal capacitor, show that the average power supplied by thesource over a complete cycle is zero.(ii) A bulb is connected in series with a variable capacitor and an A.C. source as shown. What happens tothe brightness of the bulb when the key is plugged in and capacitance of the capacitor is gradually reduced

2. Predict the directions of induced currents in metal rings 1 & 2 lying in the same plane where current I in the wire is increasing steadily?

2. (a) When a bar magnet is pushed towards (or away) from the coil connected to a galvanometer, thepointer in the galvanometer deflects. Identify the phenomenon causing this deflection and write the factorson which the amount and direction of the deflection depends. State the laws describing this phenomenon.(b) Sketch the change in flux, emf and force when a conducting rod PQ of resistance R and length l movesfreely to and fro between A and C with speed υ on a rectangular conductor placed in uniform magnetic fieldas shown in the figure.

OR

In a series LCR circuit connected to an a.c. source of voltage v = vm sinωt, use phasor diagram to derive

an expression for the current in the circuit. hence obtained the expression for the power dissipated in

the circuit. Show that power dissipated at resonance is maximum.

CBSE 2015

1. A planar loop of rectangular shape is moved within the region of a uniform magnetic field acting perpendicular

to its plane. What is the direction and magnitude of the current induced in it ?

2. Sunita and her friends visited an exhibition. The policeman asked them to pass through a metal detector.

Sunita’s friends were initially scared of it. Sunita, however, explained to them the purpose and working of

the metal detector.

Answer the following questions :

(a) On what principle does a metal detector work ?

(b) Why does the detector emit sound when a person carrying any metallic object walks through it ?

(c) State any two qualities which Sunita displayed while explaining the purpose of walking through the

detector.

3. (a) State Faraday’s law of electromagnetic induction.(b) Explain, with the help of a suitable example, how we can show that Lenz’s law is a consequence of theprinciple of conservation of energy.(c) Use the expression for Lorentz force acting on the charge carriers of a conductor to obtain theexpression for the induced emf across the conductor of length l moving with velocity v through a magneticfield B acting perpendicular to its length.

OR (a) Using phasor diagram, derive the expression for the current flowing in an ideal inductor connected to

an a.c. source of voltage, V = Vo sin ωt. Hence plot graphs showing variation of (i) applied voltage and(ii) the current as a function of ωt.

(b) Derive an expression for the average power dissipated in a series LCR circuit.

CBSE 2014

1. Why is the use of AC voltage preferred over DC voltage? Give two reasons.

2. The electric current flowing in a wire in the direction from B to A is decreasing. Find out the direction of the induced current in the metallic loop kept above the wire as shown.

3. Define the term ‘mutual inductance’ between the two coils. Obtain the expression for mutual inductance of a pair of long coaxial solenoids each of length l and radii r1 and r2 (r2 >> r1). Total number of turns in the two solenoids are N1 and N2 respectively.

4. A voltage V = V0 sin ωt is applied to a series LCR circuit. Derive the expression for the average power dissipated over a cycle. Under what condition is

(i) no power dissipated even though the current flows through the circuit, (ii) maximum power dissipated in the circuit?

5. Define the term self-inductance of a solenoid. Obtain the expression for the magnetic energy stored in an inductor of self-inductance L to build up a current I through it.

6. (a) A rod of length l is moved horizontally with a uniform velocity ‘v’ in a direction perpendicular to its length through a region in which a uniform magnetic field is acting vertically downward. Derive the expression for the emf induced across the ends of the rod. (b) How does one understand this motional emf by invoking the Lorentz force acting on the free charge carriers of the conductor? Explain.

CBSE 2013

1. How does the mutual inductance of a pair of coils change when (i) distance between the coils is increased and (ii) number of turns in the coils is increased?

2. The motion of copper plate is damped when it is allowed to oscillate between the two poles of a magnet. What is the cause of this damping?

3. A light metal disc on the top of an electromagnet is thrown up as the current is switched on. Why? Give reason.

4. (a) For a given a.c., i = im sin ωt, show that the average power dissipated in a resistor R over a complete

cycle is

(b) A light bulb is rated at 100 W for a 220 V a.c. supply. Calculate the resistance of the bulb. 5. A rectangular conductor LMNO is placed in a uniform magnetic field of 0.5 T. The field is directed

perpendicular to the plane of the conductor. When the arm MN of length of 20 cm is moved towards left

with a velocity of 10 ms–1, calculate the emf induced in the arm. Given the resistance of the arm to be 5 W (assuming that other arms are of negligible resistance) find the value of the current in the arm.

6. A wheel with 8 metallic spokes each 50 cm long is rotated with a speed of 120 rev/min in a plane normal to the horizontal component of the Earth’s magnetic field. The Earth’s magnetic field at the plane is 0.4 G and the angle of dip is 60°. Calculate the emf induced between the axle and the rim of the wheel. How will the value of emf be affected if the number of spokes were increased?

CBSE 2012

1. What is the direction of induced current in coil PQ and CD?

2. Mention the two characteristic properties of the material suitable for making core of a transformer.

3. State the underlying principle of a transformer. How is the large scale transmission of electric energy over long distances done with the use of transformers?

4. A light bulb is rated 100 W for 220 V ac supply of 50 Hz. Calculate (i) the resistance of the bulb; (ii) the rms current through the bulb

OR

An alternating voltage given by V = 140 sin 314 t is connected across a pure resistor of 50 W. Find

(i) the frequency of the source.

(ii) the rms current through the resistor.

5. A series LCR circuit is connected to an ac source. Using the phasor diagram, derive the expression for the impedance of the circuit. Plot a graph to show the variation of current with frequency of the source, explaining the nature of its variation.

CBSE 2011

1. Predict the polarity of the capacitor in the situation describes below :

2. State the principle of working of a transformer. Can a transformer be used to step up or step down a d.c.voltage? Justify your answer.

3. Mention various energy losses in a transformer.4. What are eddy currents? Write any two applications of eddy currents.

5. State the working of a.c. generator with the help of a labelled diagram. The coil of an a.c. generator having

N turns, each of area A, is rotated with a constant angular velocity ω . Deduce the expression for the

alternating e.m.f. generated in the coil. What is the source of energy generation in this device?

OR

(a) Show that in an a.c. circuit containing a pure inductor, the voltage is ahead of current by in phase.

(b) A horizontal straight wire of length L extending from east to west is falling with speed v at right anglesto the horizontal component of Earth’s magnetic field B.

(i) Write the expression for the instantaneous value of the e.m.f. induced in the wire.(ii) What is the direction of the e.m.f.?(iii) Which end of the wire is at the higher potential?

CBSE 2010

1. Define self-inductance of a coil. Write its SI units.

2. Two identical loops, one of copper and the other of aluminium, are rotated with the same angular speed in

the same magnetic field. Compare (i) the induced emf and (ii) the current produced in the two coils. Justify

your answer.

3. A rectangular loop and a circular loop are moving out of a uniform magnetic field to a field-free region with

a constant velocity ‘v’ as shown in the figure. Explain in which loop do you expect the induced emf to be

constant during the passage out of the field region. The magnetic field is normal to the loops.

4. Draw a schematic diagram of a step-up transformer. Explain its working principle. Deduce the expressionfor the secondary to primary voltage in terms of the number of turns in the two coils. In an idealtransformer, how is this ratio related to the currents in the two coils? How is the transformer used in largescale transmission and distribution of electrical energy over long distances?

5. State Faraday’s law of electromagnetic induction. Figure shows a rectangular conductor PQRS in which theconductor PQ is free to move in a uniform magnetic field B perpendicular to the plane of the paper. Thefield extends from x = 0 to x = b and is zero for x > b. Assume that only the arm PQ possesses resistance r.When the arm PQ is pulled outward from x = 0 to x = 2b and is then moved backward to x = 0 withconstant speed v, obtain the expressions for the flux and the induced emf. Sketch the variations of thesequantities with distance 0 x 2b.

CBSE 2009

1. (i) State Faraday’s law of electromagnetic induction.(ii) A jet plane is travelling towards west at a speed of 1800 km/h. What is the voltage difference developedbetween the ends of the wing having a span of 25 m, if the Earth’s magnetic field at the location has amagnitude of 5 X 10- 4 T and the dip angle of 30°?

2. (a) What are eddy currents? Write their two applications.(b) Figure shows a rectangular conducting loop PQRS in which arm RS of length ' l' is movable. The loop iskept in a uniform magnetic field ‘B’ directed downward perpendicular to the plane of the loop. The arm RSis moved with a uniform speed ‘v’.

Deduce an expression for : (i) the emf induced across the arm ‘RS’,(ii) the external force required to move the arm, and(iii) the power dissipated as heat.

3. (a) State Lenz’s law. Give one example to illustrate this law. “The Lenz’s law is a consequence of theprinciple of conservation of energy.” Justify this statement.(b) Deduce an expression for the mutual inductance of two long co-axial solenoids but having different radiiand different number of turns.

4. (i) State the law that gives the polarity of the induced emf. capacitor is connected to 220 V, 50 Hz source. Find the capacitive reactance and the rms (ii) A 15

current.